The invention relates to a hydraulic piston pump as set forth hereinafter.
Such reciprocating piston pumps for generating a relatively high pressure are already known from German Published, Non-Examined Patent Application DE-OS 32 36 536. During operation in the ABS system mode, a pump of this type is intended to return pressure medium taken from a wheel brake cylinder during a brake pressure reduction phase back to the main brake cylinder of a brake installation of a motor vehicle with an ABS system, such as known from U.S. Pat. No. 4,861,116. Pump pressures of up to approximately 200 bar can occur in the course of this operation. The reciprocating piston pump has an inlet valve and an outlet valve in the form of one-way valves. These valves have a conical valve seat of great precision and a hardened steel ball as a closing body, which is pressed against the valve seat by means of a helical pressure spring. While a spring with a relatively low closing force is assigned to the inlet valve, the outlet valve has a spring with a great closing force in order to diminish the bounce of the ball, which causes noise during closing of the valve.
Furthermore, very high demands are made on the tightness of particularly the outlet valve in order to prevent, on the one hand, the flow-off of pressure medium from the brake line into a pressure medium reservoir disposed at the inlet side of the pump during braking. The reservoir is intended for the intermediate storage of a pressure medium during the pressure reduction phases in the course of ABS operation, so that it can be transported by the pump. On the other hand, the tightness of the valves also prevents the automatic emptying of the pressure medium reservoir into the main brake cylinder, if during braking without ABS operation, pressure medium is incorrectly collected in the reservoir. However, this disrupts quick brake pressure reduction in the course of an anti-blocking operation.